The invention relates generally to high power fiber laser amplifiers and more particularly to polarization control of fiber laser amplifiers.
There is a significant need for high power fiber laser amplifiers, for example, in the fields of materials processing, military and scientific applications, and as sources for further power or energy scaling via beam combining methods. A high power fiber laser receives an input signal beam from a master oscillator, which may be processed by a controller, then combined with a pump laser light in a fiber amplifier to provide a high power output. The controller can be used to perform a number of adjustments to the signal beam so as to improve the performance or control the characteristics of the high power laser output, such as its phase, amplitude, or polarization state.
High power fiber laser amplifiers use optical fiber to amplify the power from the master oscillator. Light propagating in the fiber amplifier is constrained to one or more spatial “modes” defined by the waveguide of the optical fiber. It is generally desirable to confine all the amplified power from the fiber amplifier into a single mode, specifically the lowest order fundamental mode of the waveguide since this mode yields the highest spatial brightness output. Some fiber amplifier waveguides are designed to have a single mode by designing the light to be confined to very small core radii, but these fibers are typically ill-suited for kilowatt class fiber amplifiers as the high optical intensities in the fiber core result in undesirable nonlinear effects and optical damage. Hence, most fibers suitable for kW-class amps support multiple fiber modes, consisting of both the desired fundamental mode that is optimum for power amplification along with one or more higher order modes (HOMs). If light from the fundamental mode is coupled by various circumstances into any of the HOMs, it will diminish the quality of the output. Hence, a key design consideration for kW fiber amplifiers is to ensure near-100% of the output power is contained in the fundamental mode rather than in any of the HOMs, by preventing power transfer in the amplifier fiber from the fundamental mode to any HOM.
One undesirable nonlinear process that presently imposes a scaling limit on fiber amplifier output power by transferring power from the fundamental mode to HOMs is called the High Order Mode Instability (HOMI). In the HOMI process, a moving long-period grating (LPG) in the fiber core refractive index is written by the interference pattern between the fundamental mode and an HOM as explained in C. Jauregui et al, “Physical origin of mode instabilities in high power fiber laser systems,” Opt. Express 20, 12912 (2012).
Numerous experimental studies have shown that the mechanism for the refractive index change is thermal (dn/dT). As output power increases, the LPG amplitude increases, and coupling gain builds up exponentially for power transfer from the fundamental mode to the HOM, eventually reaching a threshold level above which power dynamically fluctuates between these two modes. The dynamic fluctuation in modal powers is consistently observed to occur on timescales corresponding to the thermal diffusion time across the fiber core (typically ˜ms/kHz for 20-um class core diameters in silica fiber).
Since fiber amplifiers typically use an optical fiber that is non-polarization maintaining, a polarization controller is often used to ensure near-100% of the output power is contained within a single desired state of polarization (SOP). A small dither is added to the polarization signal, and then a portion of the output of the fiber amplifier is used to provide a feedback signal to control the polarization.
It has been observed that changing the polarization of light input to the fiber amplifier abruptly triggers a conversion of the output of the fiber amp from a fundamental mode to a higher order mode, via the HOMI process. In some cases, polarization dithering has been observed to lower the threshold for the onset of high-order mode instability in kW fiber amplifiers to approximately 60-90% of total output, significantly reducing the achievable output power from fiber amps with stable polarization. In other words, it is possible to have high power or stable polarization, but not both. This mechanism is called the polarization-induced high order mode instability (PI-HOMI).
Thus, a need exists for fiber amplifiers with stable polarization control that does not trigger the PI-HOMI process and enables output up to 100% of the regular power threshold of the HOMI process.